Deficiency in the dedicator of cytokinesis-8 (DOCK8) gene, a guanine nucleotide exchange factor, results in autosomal recessive Hyper-IgE syndrome characterized by recurrent pulmonary infections, eczema, cutaneous viral infections, elevated serum IgE levels, food allergy, low numbers of T cells, and deficient antibody responses. We have found that DOCK8 deficient patients have impaired IgG antibody responses, lack of memory B cells, reduced numbers of marginal zone (MZ)-like B cells and of follicular helper (TFH) cells. Unlike normal subjects, they make IgE antibodies to pneumococcal antigens. We also made the novel observation that DOCK8 functions downstream of TLR9 in B cells, and is essential for TLR9-induced activation of STAT3, which plays an important role in the differentiation of B cells into antibody producing cells, and demonstrated that DOCK8 associates with MyD88 and the tyrosine kinase Pyk2 in B cells, undergoes phosphorylation by Pyk2 and is critical for the activation of a Lyn-Syk-STAT3 cascade following TLR9 ligation in human and mouse B cells. We propose to test the overall hypothesis that DOCK8 is a key regulator of B cell activity. We will take advantage of our access to a large population of DOCK8 deficient patients, of newly constructed Dock8-/- mice whose B cells phenocopy the defects in the patients, and of mice that carry a conditional (floxed) Dock8 allele, to test the hypothesis tht DOCK8 promotes protective T dependent (TD) and type II T independent (TI) Ab. responses, but inhibits allergic IgE Ab. responses by mechanisms both intrinsic and extrinsic to B cells. We will 1) Test the hypothesis that ligand binding to TLR9 in B cells recruits and activates the tri- molecular MyD88/DOCK8/Pyk2 complex, resulting in STAT3 activation and B cell differentiation. 2) Test the hypothesis that DOCK8 cells plays a critical role in IgG antibody responses by mediating TLR9 signaling, promoting T follicular helper cell function and driving the development of MZ B cells. 3) Test the hypothesis that DOCK8 is a regulator of IgE antibody production and that IgE antibodies to microbial antigens enhance the susceptibility to infection in DOCK8 deficiency. A deeper knowledge of the role of DOCK8 in B cell function will further our understanding of the susceptibility of DOCK8 patients to infection. Since DOCK8 deficiency predisposes to autoimmunity, allergy and cancer, its study is likely to provide novel and important insights into the pathogenesis of these common conditions.